Extreme ultraviolet lithography mask

ABSTRACT

An EUV mask includes a low thermal expansion material (LTEM) substrate, a reflective multilayer (ML) above one surface of the LTEM substrate, and a conductive layer above an opposite surface of the LTEM substrate. A capping layer is provided above the reflective ML, a buffer layer is provided above the capping layer, and an absorption stack is provided above the buffer layer. The absorption stack comprises multiple layers. A multiple patterning process is performed on the absorption stack to form multiple reflective states.

PRIORITY DATA

This application is a divisional of U.S. Ser. No. 13/437,099 filed Apr.2, 2012, and entitled “Extreme Ultraviolet Lithography Process AndMask,” the entire disclosure of which is incorporated herein byreference.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experiencedexponential growth. Technological advances in IC materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generation. In the course of ICevolution, functional density (i.e., the number of interconnecteddevices per chip area) has generally increased while geometry size(i.e., the smallest component (or line) that can be created using afabrication process) has decreased. This scaling down process generallyprovides benefits by increasing production efficiency and loweringassociated costs. Such scaling down has also increased the complexity ofIC processing and manufacturing. For these advances to be realized,similar developments in IC processing and manufacturing are needed. Forexample, the need to perform higher resolution lithography processesgrows. One lithography technique is extreme ultraviolet lithography(EUVL). Other techniques include X-Ray lithography, ion beam projectionlithography, electron beam projection lithography, and multiple electronbeam maskless lithography.

The EUVL employs scanners using light in the extreme ultraviolet (EUV)region, having a wavelength of about 1-100 nm. Some EUV scanners provide4× reduction projection printing, similar to some optical scanners,except that the EUV scanners use reflective rather than refractiveoptics, i.e., mirrors instead of lenses. EUV scanners provide thedesired pattern on an absorption layer (“EUV” mask absorber) formed on areflective mask. Currently, binary intensity masks (BIM) accompanied byon-axis illumination (ONI) are employed in EUVL. In order to achieveadequate aerial image contrast for future nodes, e.g., nodes with theminimum pitch of 32 nm and 22 nm, etc., several techniques, e.g., theattenuated phase-shifting mask (AttPSM) and the alternatingphase-shifting mask (AltPSM), have been developed to obtain resolutionenhancement for EUVL. But each technique has its limitation needed to beovercome. For example, for AltPSM, one of the methods to generate aphase-shifting region without much attenuation in reflectivity is tocreate a step of appropriate height on a substrate and then form amultilayer (ML) over the step. However, the ML tends to smooth out thestep height, leading to a large transition area between phase-shiftingand non-phase-shifting regions. This will limit the achievableresolution limit. So it is desired to have further improvements in thisarea.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1. is a block diagram of a lithography process for implementing oneor more embodiments of the present invention.

FIG. 2. is a diagrammatic perspective view of a projection optics box(POB) employed in the lithography process for implementing one or moreembodiments of the present invention. Since a POB by reflective opticsis difficult to sketch, the equivalent refractive optics is used toillustrate the underlying principle.

FIG. 3-5. are diagrammatic cross-sectional views of various aspects ofone embodiment of an EUV mask at various stages of a lithography processconstructed according to aspects of the present disclosure.

FIG. 6. is a diagrammatic perspective view of an EUV mask according toaspects of the present disclosure.

FIG. 7. is a diagrammatic cross-sectional views of various aspects ofone embodiment of an EUV mask at various stages of a lithography processconstructed according to aspects of the present disclosure

FIG. 8. is a diagrammatic perspective view of another EUV mask accordingto aspects of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. For example, if the device in the figures is turned over,elements described as being “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the exemplary term “below” can encompass both an orientation ofabove and below. The apparatus may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein may likewise be interpreted accordingly.

Referring to FIG. 1, an EUV lithography process 10 that may benefit fromone or more embodiments of the present invention is disclosed. The EUVlithography process 10 employs an EUV radiation source 20 having awavelength of about 1-100 nm.

The EUV lithography process 10 also employs an illuminator 30. Theilluminator 30 may comprise refractive optics, such as a single lens ora lens system having multiple lenses (zone plates) and/or reflectiveoptics, such as a single mirror or a mirror system having multiplemirrors in order to direct light from the radiation source 20 onto themask 40. In the EUV wavelength range, reflective optics is employedgenerally. Refractive optics, however, can also be realized by e.g.,zoneplates. In the present embodiment, the illuminator 30 is set up toprovide a on-axis illumination (ONI) to illuminate the mask 40. In ONI,all incoming light rays incident on the mask at the same angle ofincidence (AOI), e.g., AOI=6°, as that of the chief ray. In realsituation, there may be some angular spread of the incident light. Forexample, if a disk illumination (i.e., the shape of the illumination onthe pupil plane being like a disk centered at the pupil center) of asmall partial coherence σ, e.g., σ=0.3, is employed, the maximum angulardeviation from the chief ray is sin⁻¹[m×σ×NA], where m and NA are themagnification and numerical aperture, respectively, of the imagingsystem (i.e., the projection optics box (POB) 50 to be detailed below).Partial coherence σ can also be used to describe a point source whichproduces a plane wave illuminating the mask 40. In this case, thedistance from the pupil center to the point source in the pupil plane isNA×σ and the AOI of the corresponding plane wave incident on the mask 40is sin⁻¹[m×σ×NA]. In the present embodiment, it is sufficient to employa nearly ONI consists of point sources with σ less than 0.3.

The EUV lithography process 10 also employs a mask 40 (in the presentdisclosure, the terms of mask, photomask, and reticle are used to referto the same item). The mask 40 can be a transmissive mask or areflective mask. In the present embodiment, the mask 40 is a reflectivemask such as described in further detail below. The mask 40 mayincorporate other resolution enhancement techniques such asphase-shifting mask (PSM) and/or optical proximity correction (OPC).

The EUV lithography process 10 also employs a POB 50. The POB 50 mayhave refractive optics or reflective optics. The radiation reflectedfrom the mask 40 (e.g., a patterned radiation) is collected by the POB50. The POB 50 may include a magnification of less than one (therebyreducing the patterned image included in the radiation).

Referring to FIG. 2, an incident light ray 60, after being reflectedfrom the mask 40, is diffracted into various diffraction orders due topresence of these mask patterns, such as a 0-th diffraction order ray61, a −1-st diffraction order ray 62 and a +1-st diffraction order ray63. For lithographic imaging, purely coherent illumination is generallynot employed. Disk illumination with partial coherence σ being at most0.3 generated by the illuminator 30 is employed. In the depictedembodiment, the non-diffracted light rays 61 are mostly (e.g., more than70%) removed by, e.g., central obscuration in the pupil plane. The −1-stand +1-st diffraction order rays, 62 and 63, are collected by the POB 50and directed to expose a target 70. Since the strength of the −1-st and+1-st diffraction order rays, 62 and 63, are well balanced, theyinterfere with each other and will generate a high contrast aerialimage. Also, the −1-st and +1-st diffraction order rays, 62 and 63, areof the same distance from the pupil center in the pupil plane, and depthof focus (DOF) is maximized.

The target 70 includes a semiconductor wafer with a photosensitive layer(e.g., photoresist or resist), which is sensitive to the EUV radiation.The target 70 may be held by a target substrate stage. The targetsubstrate stage provides control of the target substrate position suchthat the image of the mask is scanned onto the target substrate in arepetitive fashion (though other lithography methods are possible).

The following description refers to the mask 40 and a mask fabricationprocess. The mask fabrication process includes two steps: a blank maskfabrication process and a mask patterning process. During the blank maskfabrication process, a blank mask is formed by deposing suitable layers(e.g., multiple reflective layers) on a suitable substrate. The blankmask is patterned during the mask patterning process to have a design ofa layer of an integrated circuit (IC) device (or chip). The patternedmask is then used to transfer circuit patterns (e.g., the design of alayer of an IC device) onto a semiconductor wafer. The patterns can betransferred over and over onto multiple wafers through variouslithography processes. Several masks (for example, a set of 15 to 30masks) may be used to construct a complete IC device.

In general, various masks are fabricated for being used in variousprocesses. Types of EUV masks include binary intensity mask (BIM) andphase-shifting mask (PSM). An example BIM includes an almost totallyabsorptive region (also referring to as an opaque region) and areflective region. In the opaque region, an absorber is present and anincident light beam is almost fully absorbed by the absorber. Theabsorber can be made of materials containing chromium, chromium oxide,chromium nitride, aluminum-copper, titanium, titanium nitride, tantalum,tantalum oxide, tantalum nitride, and tantalum boron nitride. In thereflective region, the absorber is removed and the incident light isreflected by a multilayer (ML), which will be described in furtherdetail below. A PSM includes an absorptive region and a reflectiveregion. A portion of the incident light reflects from the absorptiveregion with a proper phase difference with respect to a light reflectedfrom the reflective region to enhance the resolution and imagingquality. The absorber of the PSM can be made by materials such astantalum nitride and tantalum boron nitride at a specific thickness. ThePSM can be attenuated PSM (AttPSM) or alternating PSM (AltPSM). AnAttPSM usually has 2%-15% of reflectivity from its absorber, while anAltPSM usually has larger than 50% of reflectivity from its absorber.

Referring to FIG. 3, a blank EUV mask 100 comprises a substrate 110 madeof low thermal expansion material (LTEM). The LTEM material may includeTiO₂ doped SiO₂, and/or other low thermal expansion materials known inthe art. The LTEM substrate 110 serves to minimize image distortion dueto mask heating. In the present embodiment, the LTEM substrate includesmaterials with a low defect level and a smooth surface. In addition, aconductive layer 105 may be deposed under (as shown in the figure) theLTEM substrate 110 for the electrostatic chucking purpose. In anembodiment, the conductive layer 105 includes chromium nitride (CrN),though other compositions are possible.

A reflective multilayer (ML) 120 is deposed over the LTEM substrate 110.According to Fresnel equations, light reflection will occur when lightpropagates across the interface between two materials of differentrefractive indices. The reflected light is larger when the difference ofrefractive indices is larger. To increase the reflected light, one mayalso increase the number of interfaces by deposing a multilayer ofalternating materials and let lights reflected from different interfacesinterfere constructively by choosing appropriate thickness for eachlayer inside the multilayer. However, the absorption of the employedmaterials for the multilayer limits the highest reflectivity that can beachieved. The ML 120 includes a plurality of film pairs, such asmolybdenum-silicon (Mo/Si) film pairs (e.g., a layer of molybdenum aboveor below a layer of silicon in each film pair). Alternatively, the ML120 may include molybdenum-beryllium (Mo/Be) film pairs, or any materialthat is highly reflective at EUV wavelengths can be utilized for the ML120. The thickness of each layer of the ML 120 depends on the EUVwavelength and the incident angle. The thickness of the ML 120 isadjusted to achieve a maximum constructive interference of the EUV lightreflected at each interface and a minimum absorption of the EUV light bythe ML 120. The ML 120 may be selected such that it provides a highreflectivity to a selected radiation type/wavelength. A typical numberof film pairs is 20-80, however any number of film pairs is possible. Inan embodiment, the ML 120 includes forty pairs of layers of Mo/Si. EachMo/Si film pair has a thickness of about 7 nm, with a total thickness of280 nm. In this case, a reflectivity of about 70% is achieved.

A capping layer 130 is formed above the ML 120 to prevent oxidation ofthe ML. In the present embodiment, the capping layer 130 includessilicon with about 4-7 nm thickness. A buffer layer 140 is formed abovethe capping layer 130 to act as an etching stop layer in a patterning orrepairing process of an absorption layer, which will be described later.The buffer layer 140 has different etching characteristics from theabsorption layer. The buffer layer 140 includes ruthenium (Ru), Rucompounds such as RuB, RuSi, chromium (Cr), Cr oxide, and Cr nitride. Alow temperature deposition process is often chosen for the buffer layerto prevent inter-diffusion of the ML 120. In the present embodiment, thebuffer layer 140 includes chromium with about 2-5 nm thickness. In oneembodiment, the capping layer and the buffer layer is a single layer.

In the present embodiment, an absorption stack 150 is formed above thebuffer layer 140. The absorption stack 150 preferably absorbs radiationin the EUV wavelength range projected onto a patterned EUV mask 200. Theabsorption stack 150 includes multiple film layers from a group ofchromium, chromium oxide, titanium nitride, tantalum nitride, tantalum,titanium, or aluminum-copper, palladium, tantalum nitride, aluminumoxide, molybdenum (Mo), or other suitable materials. With a properconfiguration of multiple film layers, the absorption stack 150 willprovide process flexibility in a subsequent etching process by differentetch characteristics of each film. In one embodiment, the absorptionstack 150 is formed by a first Mo layer 151/a first Cr layer 152/asecond Mo layer 153, as shown in FIG. 3. In the depicted embodiment, thethickness of the first Cr layer 152 is about 3 nm. The thickness of thefirst Mo layer 151 is around 41 nm and the thickness of the second Molayer 153 is around 44 nm.

One or more of the layers 105, 120, 130, 140 and 150 may be formed byvarious methods, including physical vapor deposition (PVD) process suchas evaporation and DC magnetron sputtering, a plating process such aselectrode-less plating or electroplating, a chemical vapor deposition(CVD) process such as atmospheric pressure CVD (APCVD), low pressure CVD(LPCVD), plasma enhanced CVD (PECVD), or high density plasma CVD (HDPCVD), ion beam deposition, spin-on coating, metal-organic decomposition(MOD), and/or other methods known in the art. The MOD is a depositiontechnique by using a liquid-based method in a non-vacuum environment. Byusing MOD, a metal-organic precursor, dissolved in a solvent, isspin-coated onto a substrate and the solvent is evaporated. A vacuumultraviolet (VUV) source is used to convert the metal-organic precursorsto constituent metal elements.

Referring to FIG. 4, in one of the present embodiments, the absorptionstack 150 is patterned to form the design layout pattern EUV mask 200with three states on the blank mask 100. The absorption stack 150 ispatterned to form the state 210 first by patterning technique. Apatterning process may include resist coating (e.g., spin-on coating),soft baking, mask aligning, exposure, post-exposure baking, developingthe resist, rinsing, drying (e.g., hard baking), other suitableprocesses, and/or combinations thereof. Alternatively, thephotolithography exposing process is implemented or replaced by otherproper methods such as maskless photolithography, electron-beam writing,direct-writing, and/or ion-beam writing

An etching process is followed to remove portions of the absorptionstack 150 and form the first state 210. The etching process may includedry (plasma) etching, wet etching, and/or other etching methods. In thepresent embodiment, a multiple-step dry etching is implemented. Theplasma etching starts to remove the second Mo layer 153 byfluorine-based gas, then proceeds to a second etching step to remove thefirst Cr layer 152 by chlorine-based gas, and then proceeds to a thirdetching step to remove the first Mo layer 151 by fluorine-based gas. Dueto the nature of plasma fluorine-based gas and chlorine-based gas, thefirst and third etching steps have a substantially high etchingselectivity with respect to Cr film and the second etching step has asubstantially high etching selectivity with respect to Mo film. Thusduring each etching step, the Mo and Cr layers can serve as adequateetching stop layers to each other to achieve a complete etching with agood process window.

Referring to FIG. 5, a second state 220 on the EUV mask 200 is formed byother patterning and etching processes, similar in many respects tothose discussed above in association with the formation of the firststate 210 except only the second Mo layer 153 is removed in the etchingprocess.

Still referring to FIG. 5, now the EUV mask 200 comprises three states,210, 220 and 230. The reflection coefficients of state 210, state 220,and state 230 are r1, r2 and r3, respectively by a proper configurationof each layer of the absorption stack 150, such as film material andfilm thickness, three states can achieve prespecified values of thereflection coefficients. In one embodiment, the first state 210 isconfigured as (in order from top to bottom) the buffer layer 140/thecapping layer 130/the ML 120/the LTEM substrate 110, the second state220 is configured as 3-nm Cr/41-nm Mo/the buffer layer 140/the cappinglayer 130/the ML 120/the LTEM substrate 110, and the third state 230 isconfigured as 44-nm Mo/3-nm Cr/41-nm Mo/the buffer layer 140/the cappinglayer 130/the ML 120/the LTEM substrate 110. The three states areconfigured such that the absolute value of r1 is larger than theabsolute value of r2 and the absolute value of r2 is larger than theabsolute value of r3. The difference of the phases of the reflectioncoefficients of the state 210 and the state 220 is 180° (referred to asbeing out of phase). Meantime, the difference of the phases of thereflection coefficients of the state 210 and the state 230 is 360°(referred to as being in phase). The state 210 and state 220 of the EUVmask 200 are assigned to adjacent polygons 310 and 320. The state 230 isassigned to the field 330, which represents the background region on themask without polygons), as shown in FIG. 6. (Note that the phase q of acomplex number r is cos⁻¹[a/(a²+b²)^(0.5)]×(180/π) if b≧0 and−cos⁻¹[a/(a²+b²)^(0.5)]×(180/π) if b<0, where a and b are the real partand the imaginary part of the complex number r, respectively. Also, notethat phase q is the same as phase q±360°×i, i being an integer.)

In another embodiment, three states 210, 220 and 230 are configured sothat all three states are in phase. The difference of the phases of thereflection coefficients of any two states are less than 40°. As anexample, the state 210 is configured as (in order from top to bottom)the buffer layer 140/the capping layer 130/the ML 120/the LTEM substrate110, the state 220 is configured as 88-nm Mo/3-nm Cr/85-nm Mo/the bufferlayer 140/the capping layer 130/the ML 120/the LTEM substrate 110, andthe state 230 is configured as 3-nm Cr/85-nm Mo/the buffer layer 140/thecapping layer 130/the ML 120/the LTEM substrate 110. The three statesare configured such that the absolute value of r1 is larger than theabsolute value of r3 and the absolute value of r3 is larger than theabsolute value of r2. The state 210 and 220 are assigned to adjacentpolygons, 310 and 320. The state 230 is assigned to the field 330, asshown in FIGS. 7 and 8.

The present disclosure is directed towards lithography systems andprocesses. In one embodiment, an extreme ultraviolet lithography (EUVL)process includes receiving an extreme ultraviolet (EUV) mask withmultiple states, wherein different states of the EUV mask are assignedto adjacent polygons and a field (a region without polygons), exposingthe EUV mask by a nearly on-axis illumination (ONI) with partialcoherence σ less than 0.3 to produce diffracted lights andnon-diffracted lights, removing most of the non-diffracted lights,collecting and directing the diffracted lights and the not removednon-diffracted lights by a projection optics box (POB) to expose atarget.

In another embodiment, an EUVL process includes forming an EUV mask withthree states, assigning different states of the EUV mask to adjacentpolygons and a field, exposing the EUV mask by nearly an on-axisillumination (ONI) with partial coherence σ less than 0.3 to producediffracted lights and non-diffracted lights, removing more than 70% ofthe non-diffracted lights, and collecting and directing the diffractedlights and the not removed non-diffracted lights by a projection opticsbox (POB) to expose a semiconductor wafer

The present disclosure is also directed towards masks. In oneembodiment, an EUV mask includes a low thermal expansion material (LTEM)substrate, a reflective multilayer (ML) above one surface of the LTEMsubstrate, and a conductive layer above an opposite surface of the LTEMsubstrate. A capping layer is provided above the reflective ML, a bufferlayer is provided above the capping layer, and an absorption stack isprovided above the buffer layer. The absorption stack comprises multiplelayers. A multiple patterning process is performed on the absorptionstack to form multiple states, which are assigned to different polygonsand a field.

Based on the above, it can be seen that the present disclosure offersthe EUV lithography process 10. The EUV lithography process 10 employs anearly ONI, e.g., a disk illumination with partial coherence σ smallerthan 0.3 to expose an EUV mask to produce diffracted lights andnon-diffracted lights. The EUV lithography process 10 removes more than70% of the non-diffracted lights and utilizes mainly the diffractedlights from two symmetrically located (on the pupil plane) and intensitybalanced −1-st and +1-st diffraction orders to expose a semiconductorwafer. The EUV lithography process 10 also employs an EUV mask withthree states with prespecified reflection coefficients. Different statesare assigned to adjacent polygons and a field. The EUV lithographyprocess 10 demonstrates an enhancement of aerial image contrast for bothline/space and end-end patterns, and achieves a high depth of focus(DOF) in a large pitch range. The EUV lithography process 10 provides aresolution enhancement technique for future nodes.

The foregoing outlined features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. An extreme ultraviolet lithography (EUVL) mask,comprising: a low thermal expansion material (LTEM) substrate; areflective multilayer (ML) above one surface of the LTEM substrate; acapping layer above the reflective ML; a buffer layer above the cappinglayer; and an absorption stack above the buffer layer, wherein theabsorption stack comprises three layers that are patterned to provide atleast two openings, wherein a first of the openings exposes a topsurface of the buffer layer and a second of the openings exposes a topsurface of a layer of the absorption stack.
 2. The EUVL mask of claim 1,wherein the first of the openings provides a reflective coefficient ofr1, the second of the openings provides a reflective coefficient of r2,and the absorption stack provides a reflective coefficient of r3, andwherein said three reflective coefficients are configured as an absolutevalue of r1 is larger than an absolute value of r2 and the absolutevalue of r2 is larger than an absolute value of r3.
 3. The EUVL mask ofclaim 2, wherein a difference of phases of r1 and r2 is in a range from160° to 200°, a difference of phases of r2 and r3 is in a range from160° to 200°, and a difference of phases of r3 and r1 is less than 40°.4. The EUVL mask of claim 1, wherein a first reflective state isprovided by layers of the absorption stack that remain under the secondof the openings, at least one of the buffer layer and the capping layer,the reflective ML, and the LTEM substrate.
 5. The EUVL mask of claim 4,wherein a second reflective state is provided by the three layers of theabsorption stack, at least one of the buffer layer and the cappinglayer, the reflective ML, and the LTEM substrate; and a third reflectivestate is provided by the buffer layer, the capping layer, the reflectiveML, and the LTEM substrate.
 6. An extreme ultraviolet lithography (EUVL)system for patterning a target, comprising: an extreme ultraviolet (EUV)mask having an absorption stack disposed over a reflective multilayer(ML), wherein the absorption stack provides multiple reflective statesincluding a first reflective state and a second reflective state,wherein the first reflective state is provided by an exposed top surfaceof a first absorption stack layer and the second reflective state isprovided by an exposed top surface of a second absorption stack layer;an EUV light source operable to provide an incident beam on the EUV maskof a nearly on-axis illumination (ONI); a projection optics box (POB)operable to collect and direct a reflected, diffracted light from themultiple reflective states of the EUV mask.
 7. The system of claim 6,wherein the EUV mask further comprises: a low thermal expansion material(LTEM) substrate wherein the reflective ML is disposed above one surfaceof the LTEM substrate; a conductive layer above an opposite surface ofthe LTEM substrate; a capping layer above the reflective ML; and abuffer layer above the capping layer; and wherein the absorption stackis above the buffer layer.
 8. The system of claim 7, wherein the cappinglayer and the buffer layer are a single material layer.
 9. The system ofclaim 6, wherein the first absorption layer include Cr, and wherein thesecond absorption layer includes Mo.
 10. The system of claim 6, whereina third state reflective state of the EUV mask is provided by a bufferlayer, a capping layer, the reflective ML, and the LTEM substrate. 11.The system of claim 7, wherein the second reflective state is providedby the first absorption stack layer, the second absorption stack layer,an underlying third absorption stack layer, at least one of the bufferlayer and the capping layer, the reflective ML, and the LTEM substrate.12. The system of claim 11, wherein the first absorption stack layer isa Cr layer, the second absorption stack layer is a Mo layer, and thethird absorption stack layer is a Mo layer.
 13. The system of claim 7,wherein the first reflective state is provided by the first absorptionstack layer, a third absorption stack layer, at least one of the bufferlayer and the capping layer, the reflective ML, and the LTEM substrate.14. The system of claim 13, wherein the first absorption stack layer isa Cr layer, and the third absorption stack layer is a Mo layer.
 15. Anextreme ultraviolet (EUV) lithography mask, comprising: a multilayerstructure over a mask substrate; a buffer layer over the multilayerstructure; a first absorption stack layer over the buffer layer; asecond absorption stack layer over the first absorption stack layer; anda third absorption stack layer over the second absorption stack layer;wherein the first, second, and third absorption stack layers arepatterned to provide a first reflective state at a top surface of thebuffer layer, a second reflective state at a top surface of the secondabsorption stack layer, and a third reflective state provided by a topsurface of the third absorption stack layer.
 16. The mask of claim 15,wherein the first, second and third absorption stack layers include Mo,Cr, and Mo respectively.
 17. The mask of claim 15, wherein the masksubstrate is a low thermal expansion material (LTEM) substrate.
 18. Themask of claim 15, further comprising: a capping layer interposing themultilayer structure and the buffer layer.
 19. The mask of claim 15,wherein the multilayer structure includes Mo/Si layer pairs.
 20. Themask of claim 15, wherein the first, second and third absorption stacklayers are selected from the group consisting of chromium, chromiumoxide, titanium nitride, tantalum nitride, tantalum, titanium,aluminum-copper, palladium, aluminum oxide, molybdenum (Mo), andcombinations thereof.